Kenya's police chief says about 200 Islamic extremist fighters have split from Somalia's al-Shabab rebels, who are allied to al-Qaida, and have instead pledged allegiance to the Islamic State group.

Joseph Boinett told The Associated Press that the splinter group is operating in Kenya's north, near the Somali border, and has carried out at least two attacks in the last two weeks, killing one soldier and two civilians in Mandera County.

Boinnet said the split poses an extra challenge for Kenya's security forces.

Al-Shabab has vowed retribution on Kenya for sending troops to Somalia to fight the extremists. Kenya has experienced a series of al-Shabab attacks since it sent its troops to Somalia in 2011.